P. Djemia

Mechanical properties of diamond films: A comparative study of polycrystalline and smooth fine-grained diamonds by Brillouin light scattering

Brillouin light scattering, Raman light scattering and x-ray diffraction were used to investigate the elastic and microstructural properties of polycrystalline and smooth fine-grained diamond films of varying diamond quality. They were deposited on a titanium alloy by a two-step microwave plasma-assisted chemical vapor deposition process at 600 °C. Their morphology and roughness were studied by scanning electron microscopy and atomic force microscopy. Their refractive indices were determined by the M-line spectroscopy technique. The diamond purity of all these coatings in terms of the sp3 bonding fraction was deduced from visible and UV Raman spectroscopy as a function of the deposition conditions. All the samples were found to be textured with a 〈011〉 crystallographic direction normal to the film plane, leading to essentially hexagonal symmetry of the elastic tensor. By taking advantage of the detection of a number of different acoustic modes, complete elastic characterization of the films was achieved. ...

Surface acoustic waves in the GHz range generated by periodically patterned metallic stripes illuminated by an ultrashort laser pulse

The photoelastic response of periodic arrays of stripes attached to the surface of a substrate and illuminated by an ultrashort laser pulse were investigated. The samples were gold arrays on silicon and aluminum arrays either on crystalline quartz or on silicon. The metallic stripes had submicrometer lateral dimensions and the spatial periods ranged from about 1 microm up to 5 microm. The substrate being transparent (quartz) or slightly absorbing (silicon) at the laser wavelength (lambda = 750 nm), a laterally modulated thermal stress is generated near the surface of the substrate when a light pulse illuminates the structure. The studies of vibrations involved by the subsequent relaxation processes show that surface acoustic waves at frequency as high as about 5 GHz are excited with the samples consisting of aluminum stripes. In the case of the aluminum samples with the largest lateral spatial periods (aluminum on quartz), the surface acoustic wave propagates outside the illuminated area. In the case of the gold samples, a normal mode of individual bars is observed instead. Experimental evidence shows that these behaviors are mainly governed both by the lateral spatial period of the structure and by the density of the metal.

Elastic-strain distribution in metallic film-polymer substrate composites

Synchrotron x-ray radiation was used for in situstrain measurements during uniaxial tests on polymer substrates coated by a metallic goldfilm 400 nm thick deposited without interlayer or surface treatment. X-ray diffraction allowed capturing both components elastic strains and determining how these were partitioned between the metallic film and the polymeric substrate. For strains below 0.8%, deformation is continuous through the metal-polymer interface while above, the onset of plasticity in the metallic film induces a shift between film and substrate elastic strains.

Nondestructive evaluation of micrometric diamond films with an interferometric picosecond ultrasonics technique

Synthetic diamond thin or thick films are suitable materials for many applications because of their weak friction coefficient and their large hardness, Young modulus, and thermal conductivity. A series of diamond thin films of variable quality obtained by plasma-assisted chemical vapor deposition lying on titanium alloy substrates has been studied by picosecond ultrasonics. Femtosecond laser pulses allow the generation and detection of picosecond acoustic pulses that can be used to study the elastic properties of thin micrometric and submicrometric films. Acoustic fields generated in the substrate and propagating in the transparent diamond film are detected by an interferometric technique; they give rise to oscillations and abrupt step-like changes in the transient reflectivity variations. An analysis of these different features allows the determination of various characteristics of these films, such as longitudinal elastic constant, thickness, and roughness in terms of the diamond quality.

Hardness and elasticity in cubic ruthenium dioxide

The Knoop hardness of the highly incompressible cubic phase of ruthenium dioxide was found to be 19–20 GPa from indentation tests. This value scales well with the shear modulus approximated by the elastic constant C44 of 144 GPa obtained from Brillouin scattering measurements. This work provides evidence that the shear modulus is a better indicator of hardness than the bulk modulus for ionic and covalent materials.

Ab initio calculation of the elastic properties and the lattice dynamics of the ZnxCd1−xSe alloy

The lattice dynamics and elastic properties of the ternary ZnxCd1−xSe alloy have been studied using first-principle calculations. These are done using the density-functional perturbation theory (DFPT) within local density approximation (LDA) and employing virtual-crystal approximation (VCA). We study the variation of the optical phonon frequencies (ωTO and ωLO), the high-frequency dielectric coefficient (e∞), the dynamic effective charge (Z*) and the elastic constants (C11, C12, C44) as a function of the composition (x). We found that the ωTO and ωLO follow a quadratic law in x and agree well with the experiment. The obtained e∞, Z*, C11, C12 and C44 have a quadratic form with x. The elastic constant tensors for simple cubic eight-atom supercells with x = 0.25, 0.5 and 0.75 have been computed and are in good agreement with those obtained from VCA.

Brillouin light scattering observation of the transition from the superparamagnetic to the superferromagnetic state in nanogranular (SiO2)Co films

Evolution of magnetic excitations from purely magnetostatic modes to dipole-exchange spin waves in the Damon–Eshbach geometry in nanocomposite (SiO2)100−xCox (50 at. %<x<80 at. %) films during a transition from superparamagnetic to superferromagnetic state has been studied by Brillouin light scattering. It has been shown that removal of the degeneracy of backward volume magnetostatic waves due to nonzero exchange constant manifests itself in more pronounced Stokes/anti-Stokes asymmetry. Accompanying static characterization by means of magnetic force microscopy imaging and grazing incidence small angle neutron scattering has demonstrated the presence of weak stripe domains in the superferromagnetic phase.

Surface and bulk characterizations of CNx thin films made by r.f. magnetron sputtering

Abstract Carbon nitride thin films deposited by reactive RF magnetron sputtering were studied to obtain knowledge for various applications. Different types of analyses were performed on CN x layers to determine both surface and bulk properties of the films and particular attention was paid to nitrogen incorporation in the films. The variation in composition and type of chemical bonds between atoms in the films was followed using X-ray photoelectron spectroscopy (XPS). The results showed that the N/C ratio reaches 0.7. The wettability tests can lead to the determination of superficial energy, providing very interesting information about biocompatibility. The wettability and superficial energy decreased with nitrogen content. The evolution of the elastic properties of the CN x layers as a function of the nitrogen incorporation was investigated by Brillouin spectroscopy. The measured Rayleigh wave velocity decreased with nitrogen incorporation.

Brillouin scattering from the icosahedral quasicrystal Al70.4Mn8.4Pd21.2

Abstract We report on acoustic surface waves in quasicrystal Al 70.4 Mn 8.4 Pd 21.2 studied using Brillouin spectroscopy. Our data are fitted with a spectral density modelization and our results are compared to those obtained from previous inelastic neutron scattering and ultrasonic propagation measurements on the bulk samples Al 68.7 Mn 9.6 Pd 21.7 .

Ab initio calculation of the lattice dynamics of the Boron group-V compounds under high pressure

High-pressure effects on the lattice dynamics and dielectric properties of the BN, BP, BAs, BSb and BBi alloys have been carried out using the density-functional perturbation theory within the local density approximation. We study the variation of the optical phonon frequencies (ωTO and ωLO), the high-frequency dielectric coefficient (ε∞) and the dynamic effective charge (Z*) with pressure. The ωTO and ωLO have a quadratic form with pressure for all boron compounds. The obtained ε∞ and Z* for BN, BP remain constant with pressure. However, for BAs, BSb and BBi, ε∞ and Z* have a quadratic form with pressure. Our results are in good agreement with the available experimental data for BN and BP and they allow prediction for BAs, BSb and BBi.